Gráinne

Wife of Diarmait. Gráinne was the daughter of Cormac Mac Airt, the high king of Ireland.

Gráinne only appeared in the Irish romance titled Tóraigheacht Dhiarmada agus Ghráinne (The Pursuit of Diarmait and Gráinne). The tale of the love triangle between Gráinne, Diarmait and Finn influenced other medieval romances, such as that of Tristan, Isolde and Mark, or Lancelot, Guinevere and Arthur. Some scholars do identify her with Isolde, while Diarmait was like Tristan and Finn like King Mark.

Although Gráinne was betrothed to Finn Mac Cumhaill, the aging Fian captain, she fell in love with Finn's younger companion, Diarmait. Through a geis that she imposed upon the young hero, Gráinne forced Diarmait to betray his leader, causing a feud between the two friends that would eventually cause Diarmait's death. Diarmait had no choice, since he couldn't break his geis. He eloped with Grainne.

Gráinne and Diarmait fled from Tara and lived in the mountains and wild forests, undergoing various adventures, evading the pursuit of Finn and the Fianna. Eventually peace was settled between Finn and Diarmait, neither Gráinne trusted Finn not to harm her husband, nor did Finn ever forgive Diarmait for the betrayal. Instead, it was a boar that mortally wounded Diarmait, and Finn wouldn't heal the young hero. Diarmait died of his wound.

The tale I have read ended with Diarmait's death, but some other versions said that Gráinne wanted to avenge Diarmait's death, but the shame of being forced to marry her husband's enemy and the Fianna mocking her, finally brought reconciliation between her and Finn.

See The Pursuit of Diarmait and Gráinne for the full story.